Threaded structures made of wood and other materials have been utilized for joining objects for centuries, and a variety of devices and methods have been used for producing female (or internal) and male (or external) threads in wood. Some of the history of this technology and description of currently available devices for producing wooden threads are described in my uncommonly entertaining book, The Nuts and Bolts of Woodworking.
Internal threads can be produced cross-grain in wood relatively easily utilizing metal taps generally similar to the taps used for producing threads in metal. However, machining a wooden dowel to produce an external thread is considerably more difficult, particularly on a production basis and in certain woods. One successful method uses an electric router. The basic principles associated with use of an electric router having a rotating V-shaped or veining bit or cutter for producing a thread on a dowel are described in U.S. Pat. No. 4,287,627 issued Sept. 8, 1981 to Chambers.
The first commercially available threader using an electric router was my original Beall Wood Threader, an amazingly clever device having a molded plastic base on which an electric router or laminate trimmer is mounted. Injection-molded thermoplastic inserts are mounted within the base of the original Beall Wood Threader using screws positioned parallel to the longitudinal axis of the bore through the insert that receives the dowel. While the original Beall Wood Threader works extremely well, there are certain limitations associated with its design.
These limitations are a consequence of the exacting requirements for successfully threading wooden dowels. Apparatus for this purpose must be manufactured to extremely close tolerances and, in the case of some dimensions, adjustability must be possible.
Injection molding of thermoplastic materials is a process in which it is extremely difficult to hold close tolerances. Accordingly, it is difficult to manufacture the inserts in the original Beall Wood Threader to the required tolerances. Additionally, in each of the Chambers Pat. No. 4,287,621 and the original Beall wood Threader, the highly critical location of the router (and, in turn, the rotating axis of the router bit) was achieved by use of an index sleeve temporarily positioned within the threader base and into which the router bit or cutter projected. This required that the threader inserts in turn be located very accurately relative to the threader base so that the desired relationship between the rotating axis of the router cutter and the threader insert would be achieved through the intermediate positioning of the router relative to the threader base.
While the Chambers '627 patent arguably suggests, in FIG. 5, the possibility of using inserts for threading dowels having different diameters interchangeably with approximately the same cutter height (or projection), the '627 patent suggests no way to successfully produce such a structure. Doing so is impractical using the Chambers structure, is impractical utilizing injected molded inserts and is not possible in the original Beall Wood Threader.